missing documentation: prefix hints, empty reservation, template classes, exclude-first-last-24

Certain features introduced in the 2.3 development cycle lack documentation or could be better documented:

• Prefix hints. There is no mention of what happens when a client povides a prefix hint. It should not be mentioned that Kea started respecting prefix hints with Kea 2.3.4 because that's not true. Rather, the focus was switched from matching the prefix address to matching the prefix length.
• Empty reservations. It's not mentioned that empty reservations are possible and that they can be used to quickly assign clients to KNOWN. Maybe one phrase about this would be helpful.
• Template classes. This is a big feature. However, there isn't a section dedicated to it. The info about template classes is in 2 notes and 2 paragraphs in the classification section. I think it deserves a dedicated section that users can search by.
• exclude-first-last-24. ARM mentions that this takes effects for subnets bigger than /24, but it should include /24 too - something like subnets of /24 size or smaller. Quotes from ARM:

  • Exclude First Last Addresses in Subnets Bigger Than /24

  • The exclude-first-last-24 compatibility flag is described in Configuration of IPv4 Address Pools (when true .0 and .255 addresses are excluded from subnets with prefix length less than 24).

  • When true, it applies only to subnets with prefix lengths less than 24 bits; the default is false.